Displacement measuring instruments are used for measuring the relative location of one object with reference to another. A particular application of such displacement measuring instruments can be found in machining equipment for measuring the relative location of a machine tool with respect to the bed of the machine tool. Another application can be found in coordinate measuring machines to ascertain the location and dimensions of test specimens for example.
Two categories of displacement measuring instruments are referred to as absolute and incremental. Absolute measuring systems provide a code value which represents the absolute position of a slide or a machine member directly. Discriminable codes are provided on a coding member such as a scale and a predetermined absolute origin acts as the reference so that a position can be determined solely by reading the particular code.
Incremental measuring systems produce digital signals which increase or decrease the measured value by incremental steps. A scanning device scans the coding member to generate periodic scanning signals from which counting pulses for each individual graduation, or after electronic interpolation, for a fraction of a graduation, are obtained in an evaluation device. Counting these pulses in a counter furnishes an instantaneous position measurement value and is done in each case based on freely selectable measurement reference locations. These measurement reference locations may be assigned encoded reference marks whose locations are fixedly assigned to certain positions to distinguish among them.
PCT Publication No. WO 89/11080 published Nov. 16, 1989 describes an incremental type measuring instrument. In particular, in order to generate a position-coded signal at virtually any time in a position-measuring instrument, two tracks are scanned by means of two sensors, each of which produces a sinusoidal signal pair (cos .alpha.; sin .alpha.; cos .beta., sin .beta.) or a similar periodic signal with a definite phase relation at any given time, the periods of the phases having a ratio of m/m-1. The desired relative distance is derived from the difference between the two phase relations.